Driving apparatus of a laser diode

Abstract

Disclosed is a driving apparatus of a laser diode, comprising a light detection circuit serial-connected with a pin diode detecting the intensity of the emission of light from the laser diode, the light detection circuit producing a signal depending on the intensity of the emission detected by the pin diode, a modulation circuit serial-connected with the laser diode, the output current of the modulation circuit varying depending on the difference between an input signal and the signal outputted from the light detection circuit, and a constant voltage drop device disposed in the series circuit including the laser diode and the modulation circuit, the constant voltage drop device causing a voltage drop of a certain degree regardless of the strength of a current flowing thereto. A Zener diode having Zener voltage equivalent to the dropped constant voltage or having at least one diode which generates forward voltage drop equivalent to the dropped constant voltage or the like is used as the above constant voltage drop device, the device being disposed, for example, between the laser diode and the modulation circuit.

What is claimed is: 
     
       1. A driving apparatus of a laser diode, comprising: a light detection circuit serial-connected with a pin diode detecting the intensity of the emission of light from the laser diode, said light detection circuit producing a signal depending on the intensity of the emission detected by the pin diode;   a modulation circuit serial-connected with the laser diode the output current of said modulation circuit varying depending on the difference between an input signal and said signal outputted from the light detection circuit; and   constant voltage drop means disposed in the series circuit including the laser diode and the modulation circuit, the constant voltage drop means causing a voltage drop of a certain degree regardless of the strength of a current flowing thereto.   
     
     
       2. A driving apparatus of claim 1, wherein the constant voltage drop means is a Zener diode having Zener voltage equivalent to the dropped voltage. 
     
     
       3. A driving apparatus of claim 1, wherein the constant voltage drop means is at least one diode generating a forward voltage drop equivalent to the dropped voltage. 
     
     
       4. A driving apparatus of claim 1, wherein the constant voltage drop means is disposed between the laser diode and the modulation circuit. 
     
     
       5. A driving apparatus of claim 4, wherein predetermined identical electric potential is given to a terminal of the laser diode on the side opposite to the modulation circuit and to a terminal of the pin diode on the side opposite to the light detection circuit. 
     
     
       6. A driving apparatus of claim 5, wherein the laser diode and the pin diode compose a laser diode unit having a common terminal into which said terminal of the laser diode on the side opposite to the modulation circuit and said terminal of the pin diode on the side opposite to the light detection circuit are connected to be made, the predetermined identical electric potential being given to said common terminal. 
     
     
       7. A driving apparatus of claim 6, wherein the anode of the laser diode and the cathode of the pin diode are connected to be made into the common terminal. 
     
     
       8. A driving apparatus of claim 1, wherein the modulation circuit includes an amplifier which generates voltage depending on the difference between said input signal and said output signal of the light detection circuit, and resistance which sends a current depending on the voltage generated. 
     
     
       9. A driving apparatus of claim 1, further comprising: a constant current circuit, which sends a constant current having about half strength of the maximum current driving the laser diode, wherein the modulation circuit sends both forward and backward currents whose absolute values are smaller than that of the constant current, and a current into which the current from the constant current circuit and the current from the modulation circuit are synthesized are sent to the laser diode.   
     
     
       10. A driving apparatus of claim 9, wherein the dropped voltage generated by the constant voltage drop means is set so that the electric potential of the junction of the modulation circuit and the constant current circuit can be substantially 0 volt. 
     
     
       11. A driving apparatus of claim 1, wherein the light detection circuit includes resistance connected to the pin diode and outputs a signal depending on the electric potential of the junction of the pin diode and the resistance. 
     
     
       12. A driving apparatus of a laser diode, comprising: a modulation circuit composing a first series circuit by being connected to the cathode of a laser diode, and controlling a current flowing to the laser diode; and   a light detection circuit composing a second series circuit by being connected to the anode of a pin diode, and outputting a feedback signal depending on the intensity of the emission of light from the laser diode, to the modulation circuit,   wherein the modulation circuit is driven by a plus power source and a minus power source both connected therewith, the plus power source is further connected with the anode of the laser diode and the cathode of the pin diode, and the minus power source is further connected with a terminal of the second series circuit on the side opposite the pin diode,   wherein the laser diode and the pin diode comprise a laser diode unit having a common terminal into which said anode of the laser diode and said cathode of the pin diode are connected, the plus power source being connected to the common terminal.   
     
     
       13. A driving apparatus of a laser diode, comprising: a modulation circuit composing a first series circuit by being connected to the cathode of a laser diode, and controlling a current flowing to the laser diode;   a light detection circuit composing a second series circuit by being connected to the anode of a pin diode, and outputting a feedback signal depending on the intensity of the emission of light from the laser diode, to the modulation circuit, wherein the modulation circuit is driven by a plus power source and a minus power source both connected therewith, the plus power source is further connected with the anode ofthe laser diode and the cathode of the pin diode, and the minus power source is further connected with a terminal of the second series circuit on the side opposite the pin diode; and   a constant current circuit, which sends a constant current having about half the strength of the maximum current driving the laser diode, wherein the modulation circuit sends both forward and backward currents whose absolute values are smaller than that of the constant current, and a current into which the current from the constant current circuit and the current from the modulation circuit are synthesized are sent to the laser diode.   
     
     
       14. A driving apparatus of a laser diode; comprising: a laser diode unit including a laser diode and a pin diode;   a power source for supplying a predetermined voltage to an anode of said laser diode and a cathode of said pin diode;   a light detection circuit serially connected with said pin diode and detecting the intensity of the emission of light from the laser diode, said light detection circuit producing a signal depending on the intensity of the emission detected by the pin diode;   a modulation circuit serially connected with the laser diode and outputting current depending on the difference between an input signal and said signal outputted from said light detection circuit; and   constant voltage drop means for causing a voltage drop of a certain degree regardless of the strength of a current flowing thereto, said constant voltage drop means disposed between said power source and said modulation circuit.   
     
     
       15. A driving apparatus of a laser diode, comprising: a laser diode unit including a laser diode and a pin diode;   first power source for supplying a predetermined voltage to an anode of said laser diode and a cathode of said pin diode;   a modulation circuit including a first series circuitry by being connected to the cathode of a laser diode, and controlling a current flowing to the laser diode;   a light detection circuit including a second series circuit by being connected to the anode of a pin diode, and outputting a feedback signal depending on the intensity of the emission of light from the laser diode to the modulation circuit; and   a second power source for supplying a voltage of the opposite polarity to said predetermined voltage supplied by said first power source to the anode of the pin anode.